A fuel cell converts the chemical energy into electricity. A fuel cell differs from a battery in that the fuel and oxidant of a fuel cell are supplied from sources that are external to the cell, which can generate power as long as the fuel and oxidant are supplied. A particularly useful fuel cell for powering portable electronic devices and light-duty vehicles is a direct methanol fuel cell (DMFC) in which the fuel is a liquid methanol/water mixture and the oxidant is air or oxygen. Protons are formed by oxidation of methanol and water at the anode (fuel electrode). Protons then pass through a proton-exchange membrane (PEM) from the anode to the cathode (oxidant electrode). Electrons produced at the anode in the oxidation reaction flow in the external circuit to the cathode to do useful work.
The electrochemical reactions occurring in a direct methanol fuel cell which contains an acid electrolyte may be illustrated as follows:Anode: CH3OH+H2O→CO2+6H++6e−  (1)Cathode: 3/2O2+6H++6e−→3H2O  (2)Overall: CH3OH+3/2O2→CO2+2H2O  (3)
The DMFC and other proton-exchange membrane fuel cells (PEMFCs) typically use a hydrated sheet of a perfluorinated acid-based ionomer membrane as a solid electrolyte. The electrodes each typically containing a catalyst phase (usually a thin catalyst layer) are intimately bonded to each side of the membrane. This membrane is commercially available from DuPont (under the trade name Nafion®), among several other suppliers. Many catalysts to promote methanol oxidation (Reaction 1) have been evaluated. Examples include: (1) noble metals, (2) noble metal alloys, (3) alloys of noble metals with non-noble metals, (4) chemisorbed layers on Pt, (5) platinum with inorganic material, and (6) redox catalysts. Based on literature reports, Pt—Ru appears to be the best methanol-oxidation catalyst in acidic electrolytes.
The methanol/water fed to a DMFC may be in the liquid or vapor phase. If fuel cells using liquid fuel are available in small size, they would be able to power small-sized electronic devices for a long time. However, conventional DMFCs require pumps and blowers to feed liquid fuel to the fuel cell (e.g., S. Surampudi, et al., U.S. Pat. No. 6,248,460, Jun. 19, 2001). The resulting power system is complex in structure and large in size. One way to overcome this problem is to utilize capillary action to feed liquid fuel, without using a liquid delivery pump. However, a fuel cell of this type still has the following disadvantages: (1) poor performance due to low electrode reactivity and (2) low fuel utilization efficiency due to methanol cross-over from the anode through the electrolyte membrane to the cathode. This problem of methanol crossing over without being reacted is relatively more severe in a fuel cell with a pressurizing pump than in one without a pump.
It is believed that methanol vapor cells that operate at higher temperatures are advantageous in that the step of methanol ionization to produce protons (e.g., Reaction (1)) proceeds more rapidly in these cells. Presumably, a higher temperature results in a higher catalytic electrode activity and the faster reaction leads to a reduction in fuel cross-over. However, in the conventional DMFC of a vapor feed type, methanol (as a liquid fuel) is introduced by a pump into a vaporizer which vaporizes methanol with the resulting methanol vapor then being fed to the fuel cell by a blower. Unconsumed methanol vapor discharged from the outlet of the fuel electrode is recycled to the methanol tank through a condenser. This process needs a complex system (including a pump, a vaporizer, a blower, and a condenser) and, hence, is not suitable for powering small-sized electronic devices.
Tomimatsu, et al., in U.S. Pat. No. 6,447,941 (Sep. 10, 2002), disclosed a fuel cell in the form of stacked unit cells each having a power generating section composed of a fuel electrode, an oxidant electrode, and an electrolyte plate held in-between. The unit cells are placed on top of one another. In this fuel cell stack, a liquid fuel is introduced into each unit cell by the capillary action and evaporated in each unit cell in a fuel evaporating layer, so that the fuel electrode is supplied with the evaporated fuel. This is a very interesting fuel cell design since it makes use of two sound approaches: liquid feed by capillary action and vapor state reaction. However, the fuel cell configuration is still too complex since each unit cell contains, among other layers, separate anode, liquid-permeating, and fuel evaporating layers. Furthermore, the fuel (water-methanol mixture) will continue to vaporize even at room temperature when the fuel cell is not in operation, leading to continuous parasitic energy loss.
Our co-workers (Yang and Huang, U.S. Ser. No. 10/762,626, filed Jan. 23, 2004) disclosed a highly efficient direct vapor fuel cell (DVFC) that eliminated some of the drawbacks of the design by Tamimatsu, et al. The DVFC comprises (A) an anode receiving a liquid fuel from a liquid fuel source substantially through diffusion; (B) an electrolyte plate having a first surface adjacent to the anode; and (C) a cathode adjacent to a second surface of the electrolyte plate and opposite to the anode. The anode is provided with a heating environment to at least partially vaporize the liquid fuel inside the anode and the anode further comprises a catalyst phase to ionize the fuel in a vapor form to produce protons and electrons. In both DVFC and Tamimatsu fuel cell, the fuel cell stack is in need of multiple bipolar plates, which are normally extremely thick and brittle, making the whole fuel cell system bulky.
Cropley, et al. (U.S. Pat. No. 6,811,905, Nov. 2, 2004) disclosed an interesting fuel cell structure, which features a vapor diffusion chamber being positioned in front of the anode and a vapor transport member being positioned in front of the vapor diffusion chamber. The vapor transport member is substantially impermeable to an organic fuel/water mixture in a liquid phase but is permeable to the mixture in a vapor phase. According to the disclosure, when the fuel cell is in operation, a liquid fuel mixture delivered to the vapor transport member evaporates from the vapor transport member and is delivered to the anode in vapor form. Cropley, et al. further suggested that the vapor transport member may be selected from pervaporation, permselective, and ionomeric membranes, preferably Nafion® membranes (see Lines 24-50, Column 10 of U.S. Pat. No. 6,811,905). However, there are many drawbacks or shortcomings associated with Cropley's invention, including:
(1) Within the operating temperatures (25°-60° C.) cited by Cropley, et al. (FIG. 6 and FIG. 7 of U.S. Pat. No. 6,811,905), the vapor transport membranes as suggested (e.g., Nafion®) do not allow for significant diffusion rates of the methanol-water mixture, implying that the power-generating rate of these fuel cell systems would be extremely low. Further, very little vapor would be generated at the fuel source side, since the boiling temperatures of methanol and water are 65° C. and 100° C., respectively. The pressure differential between the source side and the sink side (the vapor diffusion chamber) would be very small, providing at best a very small driving force for pervaporation;
(2) The Nafion® membrane (whether being used as a vapor transport membrane or proton exchange membrane) can not be used at a temperature higher than 80° C. for an extended period of time due to its well-known thermal instability or high propensity to get degraded irreversibly;
(3) Clearly, Cropley, et al. did not recognize the significance of operating a DMFC at a temperature higher than 100° C. DMFCs working at a higher temperature (e.g., 120°-150°) on fuel vapors have the following advantages: (a) the step of methanol ionization to produce protons (e.g., Reaction (1)) proceeds more rapidly in these cells (e.g., J. Kallo, et al. “Conductance and Methanol Crossover Investigation of Nafion membranes in a Vapor-Fed DMFC,” J. of the Electrochemical Soc., 150 (6) (2003) PP. A765-A769); (b) a higher temperature results in a higher catalytic electrode activity and the faster reaction leads to a reduction in fuel crossover; and (c) higher operation temperatures could drastically reduce or eliminate CO poisoning of platinum or possibly even allow platinum to be replaced by much less expensive catalysts; and
(4) Cropley, et al. did not recognize the issues of differential permeation rates between methanol and water through the vapor transport membrane. The water-to-methanol ratio of the fuel after permeation can be drastically different than that of the fuel before permeation. If the methanol-water mixture is delivered to the anode catalyst site at a ratio significantly different from a desirable ratio (e.g., the stoichiometric ratio as defined by the anode electro-chemical reaction), either excess water or excess methanol will be present at the anode side to still cause the fuel crossover problem. Besides, the composition (methanol-to-water ratio) of the water-methanol mixture at the fuel source side will vary with time. This would result in the methanol-to-water ratio of the fuel delivered to the anode catalyst varying with time, leading to unstable fuel cell operation and undesirable side effects.
Therefore, one object of the present invention is to provide a simple configuration for a fuel cell that operates primarily on an organic fuel vapor at a fast and well-regulated reaction rate, with significantly reduced fuel crossover.
A specific object of the present invention is to provide a fuel cell that operates on a liquid-fed methanol/water mixture fuel, which is then vaporized at a regulated rate near the anode catalyst prior to being ionized to produce protons.
A further specific object of the present invention is to provide a fuel cell that feeds on a liquid-fed methanol/water mixture but operates at a temperature higher than 100° C., preferably higher than 120° C.
Another specific object of the present invention is to provide a fuel cell that feeds on a liquid-fed methanol/water mixture at a first water-to-methanol ratio, but operates on a vapor mixture at a second water-to-methanol ratio, which is different than the first ratio. Preferably, this second ratio is closer to the balanced stoichiometric molecular ratio as defined in Eq.(1). This molecular ratio is one-to-one in the case of DMFC.
Still another specific object of the present invention is to provide a fuel cell with a stable, constant organic-to-water feed ratio of the vapor mixture that is transported to the anode catalyst phase for oxidation to produce a stable power output.